1. Field of the Invention
The present invention relates to a charging roller, and a process cartridge and an electrophotographic apparatus which each have the charging roller. More particularly, the present invention relates to a charging roller charging a surface of an electrophotographic photosensitive member at predetermined potential by applying a voltage to the charging roller which is arranged on contact with an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus which each has the charging roller.
2. Related Background Art
Heretofore, many methods are known as electrophotography methods. A general method for obtaining a duplication comprises the steps of using a photoconductive substance, forming a electric latent image on an electrophotographic photosensitive member by various means, making the latent image a visible image by performing development with toner, transferring a toner image to a transferring material such as paper if needed, and fixing the toner image, on a transferring material by heat, pressure, or the like thereafter. In addition, toner particulates which remain on the electrophotographic photosensitive member without being transferred on the transferring material are removed from the electrophotographic photosensitive member at a cleaning step.
Heretofore, as a charging device used in electrophotography, a corona charging device has been used. In recent years, the contact charging device has been put in practical use instead of this. This aims at low ozone and low power, and in particular, a roller charging method which uses an electroconductive roller as a charging member among this type of devices is preferably used in view of charging stability.
In the roller charging method, a conductive elastic roller is pressed on contact with a charged body, and the charged body is charged by applying a voltage to the conductive elastic roller.
Specifically, since charging is performed by discharge to a charged body from a charging member, charging is started by applying a voltage more than a certain threshold voltage. For example, when a charging roller is pressed on contact with an organic electrophotographic photosensitive member (OPC electrophotographic photosensitive member) which has 25-μm-thick photosensitive layer, surface potential of the electrophotographic photosensitive member starts to rise when applying a voltage of about 640 V or more in an absolute value. Thereafter, surface potential of the electrophotographic photosensitive member increases linearly at an inclination of 1 to the applied voltage. Hereafter, this threshold voltage is defined as a charging start voltage Vth.
Thus, in order to obtain a surface potential Vd of an electrophotographic photosensitive member needed for electrophotography, a DC voltage, which is higher than a voltage needed for image formation itself, such as Vd+Vth, is needed for a charging roller. A method of applying only a DC voltage in this way to a contact charging member to perform charging is called DC charging.
However, in the DC charging, since the resistance of a contact charging member is easily changed by environmental fluctuation and the like and Vth is changed when the film thickness is changed by the electrophotographic photosensitive member being worn away, it is difficult to bring the potential of the electrophotographic photosensitive member into a desired value.
For this reason, in order to achieve further equalization of charging, an AC+DC charging method of applying a voltage, obtained by superimposing an AC component having a peak-to-peak voltage of 2×Vth or more on a DC voltage equivalent to the desired Vd, to a contact charging member is used. This aims at a potential leveling effect of AC. Potential of a charged body is converged on Vd which is a center of peaks of the AC voltage, and it is not affected by disturbances such as an environment.
As a conductive member for charging, there is an example of forming a surface layer with a conductive seamless tube on a conductive support member (for example, refer to U.S. Pat. No. 4,967,231). Furthermore, a seamless tube which is made of a fluorocarbon resin is disclosed, and a multilayer tube which is constructed of layers whose conductivities are different is also disclosed. As a method concerning production as a charging member, a method of forming the charging member by insertion is mentioned as the above-mentioned conventional technology. In addition, a surface formation method using a cross head extruder is also proposed.
Even if using a foam as an elastic layer on a substrate, such a method of forming a charging roller with a seamless tube can form a uniform face by further covering the charging roller with the seamless tube. Hence, it is possible to achieve more uniform charging.
Means taken to cover a support member with a seamless tube is to achieve fitting by shrinking the tube by physical or chemical means, for example, heat with making an internal diameter of the seamless tube larger than an outer diameter of the support member to be covered, or to achieve fitting by expanding the tube by physical or chemical means, for example, air pressure with making an internal diameter of the seamless tube smaller than an outer diameter of the support member to be covered. In addition, it is also possible to use a multilayer co-molding tube (e.g., refer to Japanese Patent Application Laid-Open No. H11-125952).
As methods of giving electroconductivity to a seamless tube, there are generally an ionic conduction method of using salt as an electroconductive agent, and an electronic conduction method of using carbon black, a conductive metal oxide, metal powder, or the like as an electroconductive agent. When electroconductivity is given by the ionic conduction method, there arises a problem that environmental fluctuation of resistance becomes large easily, and that salt tends to pollute an electrophotographic photosensitive member since the charging roller abuts on the electrophotographic photosensitive member.
Nevertheless, when a contact charging device like the above is adopted as charging means of an electrophotographic apparatus which forms an electrostatic latent image by line scanning on an electrophotographic photosensitive member which is a charged body, for example, a laser beam printer, there are the following problems. When an image pattern with repetition of radiation and unradiation of a laser beam which is high density at equal intervals in a subscanning direction is outputted, a moire pattern may arise in an image face when a frequency of an AC voltage, which is applied to a contact charging member, and a spatial frequency of the image pattern become near. Although this is solvable when making the frequency of the AC voltage high enough, it becomes easy to generate vibration noise since the contact-charging member and electrophotographic photosensitive member touch. Hence, it is an extremely inconvenient defect in order to reduce noise at the time of operation of a printer or the like particularly in an office environment, or the like.
The vibration noise (hereafter, this is called a “charging noise”) in the contact charging method is caused by vibration generated by an exciting force of the AC voltage applied since the AC voltage is applied in a state that the charging member and charged body abut each other. It is considered that the vibration is caused by the charging member “patting” the charged body by the AC voltage frequency, an electric field force, and a restoring force of the elastic material. Hence, so as to reduce the charging noise, a method of making the whole charging member or an elastic material be low hardness, i.e., soft is generally adopted (e.g., refer to Japanese Patent Application Laid-Open No. H4-25868).
On the other hand, with paying attention to tan δ and storage elastic modulus in dynamic viscoelasticity measurement of an elastic layer or a surface coating film layer, there are means of enlarging tan δ (e.g., refer to Japanese Patent Application Laid-Open No. H8-262835), and means of controlling a value of tan δ and lowering a storage elastic modulus, that is, lowering hardness (e.g., refer to Japanese Patent Application Laid-Open No. H10-319676). The charging member having these features increases its silence property in comparison with the conventional ones. Nevertheless, there is still room for improvement such as easy generation of more jarring noise in a higher tone due to acceleration of the charging frequency in recent years, and easy hearing of the charging noise due to miniaturization, until it reaches a level that general users can use it satisfactorily.
Furthermore, lowering hardness or enlarging tan δ may cause problems such as a poor image and deterioration of charging noise in connection with shape deterioration due to an abutting portion of the charging roller being deformed by permanent set because of the charging roller and electrophotographic photosensitive member being kept in an abutting state for long time.